Stroke is a leading cause of long-term disability and the third leading cause of death in the US. The identification of triggers of stroke is of significant public health interest. Short-term increases in particulate air pollution may precipitate acute ischemic stroke, but previous findings have been inconsistent. This proposal will apply case-crossover methods to the study of patients hospitalized with a diagnosis of acute ischemic stroke. Specifically, we will test the hypotheses that: short-term increases in ambient particle levels can trigger acute ischemic stroke; the magnitude of this effect will be greater than previously observed when accurate information on time of symptom onset is used; and the effect of particulate matter will vary by the etiologic subtype of stroke with the greatest increase in risk to be observed in those patients with atherothrombotic and lacunar strokes. These hypotheses will be tested in patient populations with complimentary strengths.